University of Nebraska–Lincoln Extension EC1763
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How Windbreaks Work
For more windbreak publications see:
EC1763
EC1764
EC1766
EC1767
EC1768
EC1770
EC1771
EC1772
EC1777
EC1778
EC1779
How Windbreaks Work
Windbreak Establishment
Windbreaks for Livestock Operations
Windbreaks for Rural Living
Windbreak Management
Windbreak for Snow Management
Windbreaks and Wildlife
Windbreaks in Sustainable Agricultural Systems
Windbreak Renovation
Field Windbreaks
Windbreaks for Fruit and Vegetable Crops
3WEETCORNTOMATOESANDCANTALOUPEGROWINGINTHEPROTECTIONOFATWOROWMIXEDSPECIESWINDBREAKATTHE5NIVERSITYOF.EBRASKAn
,INCOLN!GRICULTURAL2ESEARCHAND$EVELOPMENT#ENTER
Extension is a Division of the Institute of Agriculture and Natural Resources at the University of Nebraska–Lincoln
cooperating with the Counties and the United States Department of Agriculture.
University of Nebraska–Lincoln Extension educational programs abide with the nondiscrimination policies of the
University of Nebraska–Lincoln and the United States Department of Agriculture.
© 2002-2006, The Board of Regents of the University of Nebraska on behalf of the University of Nebraska–Lincoln Extension. All rights reserved.
This series of windbreak publications is jointly sponsored by the University of Nebraska, USDA Natural Resources
Conservation Service, USDA Forest Service and USDA National Agroforestry Center.
University of Nebraska Extension EC 02-1763-X
How
Windbreaks
Work
By James R. Brandle, Xinhua Zhou, and
Laurie Hodges, University of Nebraska-Lincoln
Windbreaks are barriers used to reduce and redirect
wind. They usually consist of trees and shrubs but also
may be perennial or annual crops and grasses, fences,
or other materials. The reduction in wind speed behind
a windbreak modifies the environmental conditions or
microclimate in this sheltered zone.
PFRA
As wind blows against a windbreak, air pressure
builds up on the windward side (the side toward the
wind), and decreases on the leeward side (the side away
from the wind). Some of the approaching wind flows
through the windbreak, some goes around the ends,
but most of it is forced up and over the top of the windbreak. Windbreak structure — height, density, number
of rows, species composition, length, orientation and
continuity — determines which path the wind will take,
and as a result, determines how effective the windbreak
will be in reducing wind speed and altering microclimate.
A well-designed farm or ranch incorporates many types of windbreaks to protect fields, livestock, and the homesite.
Windbreak Characteristics
Effect of height, length and continuity
Windbreak height (H) is the most important factor determining the distance downwind protected by a windbreak. This value varies from windbreak to windbreak
and increases as the windbreak matures. In multiple
row windbreaks, the average height of the tallest tree
row determines the value of H. Although the height of a
windbreak determines the extent of the protected areas,
the length times the extent determines the total area
receiving protection. For maximum efficiency, the uninterrupted length of the windbreak should be at least 10
times its height.
The continuity of a windbreak also influences its
efficiency. Gaps in a windbreak become funnels that accelerate wind flow, creating areas on the downwind side
of the gap in which wind speeds often exceed open field
wind speeds. Where gaps occur, the effectiveness
of the windbreak is diminished. Lanes or field access
should be located at the end of a windbreak. If a lane
must go through a windbreak, it should be located
such that the opening is at an angle to problem
winds (Figure 1).
On the windward side of a windbreak, wind speed reductions are measurable upwind for a distance of two to
five times the height of the windbreak (2H to 5H). On the
leeward side, wind speed reductions occur up to 30H
downwind of the barrier. For example, in a windbreak
where the height of the tallest tree row is 30 feet, lower
wind speeds are measurable for 60 to 150 feet on the
windward side and up to 900 feet on the leeward side.
The magnitude of the wind reduction at any location in
the protected zone is determined by the structure of the
windbreak.
Effect of windbreak structure
Windbreak structure is made up of two components:
internal structure — the amount and arrangement
of the solid elements and open spaces; and external
structure — the cross-sectional shape of the windbreak.
The internal structural characteristics of a windbreak, especially the amount and arrangement of the
surface area and volume of the trunk, branches, and
leaves or needles, determine the magnitude of wind
speed reductions. In practice, this internal structure is
simply described in terms of density.
Windbreak density is the ratio of the solid portion of
the barrier to the total area of the barrier. As wind flows
through a windbreak, the trunk, branches and leaves
(the solid portion) absorb some of the momentum of the
wind and wind speed is reduced. In addition, as wind
flows over the tree surfaces, it is slowed by the roughness of the surface and wind speed is reduced. Together, these two processes help determine the amount of
wind speed reduction that occurs.
Around very dense windbreaks, air pressure builds
up on the windward side and a zone of low pressure develops on the leeward side. The windward air pressure
pushes air through and over the windbreak, while the
leeward low pressure area behind the windbreak pulls
air coming over the windbreak downward, creating turbulence and reducing protection downwind. As density
decreases, the amount of air passing through the windbreak increases, moderating the pressure differences
between the windward and leeward sides and reducing
the level of turbulence created by the dense windbreak.
As a result, the extent of the downwind-protected area
increases. While the extent of this protected area is
larger, the wind speed reductions are not as great as
those leeward of a more dense windbreak. By adjusting windbreak density, different wind flow patterns and
areas of protection can be established (Figure 2).
The species used and their arrangement, the number of rows and the distance between rows, and the
distance between trees are the main factors controlling
windbreak density. Increasing the number of windbreak
rows or decreasing the distance between trees increases
density and provides a more solid barrier to the wind.
Conifer species, such as cedar and pine, and shrubs
PREV
PREVAILING
WIND DIRECTION
Undesirable
Wind will funnel
through this gap
Desirable
Access lanes or roads
Figure 1. Acess lanes and roads should be at an angle to prevailing or troublesome winds. In areas where snow is a concern snow drifts may
block lane access.
with multiple stems tend to provide better year-round
density, while taller hardwood species, such as ash,
oak, or hackberry, generally are used to provide greater
height.
The interaction of height and density determines the
degree of wind speed reduction and, ultimately, the
extent of the protected area. For a windbreak with a
given height, the length of the protected area downwind
usually increases as density increases from 20 to 60
percent. At densities below 20 percent, the windbreak
provides little, if any, useful wind reduction. As densities increase above 60 percent, leeward turbulence
begins to increase, the length of the protected area
downwind begins to shrink and windbreak efficiency is
decreased.
The external structure or cross-sectional shape of
a windbreak is determined by the width, height and
arrangement of the individual tree and/or shrub rows
within the windbreak. The cross-sectional shape of
windbreaks with similar internal structures has minimal influence on wind speed reductions within 10H of
the barrier. Beyond 10H, windbreaks with a vertical
windward side tend to provide slightly more protection
than those with a slanted windward side, because more
wind passes through the barrier reducing turbulence
and extending the protected area farther to the lee.
Windbreaks with a streamlined shape in cross-section, similar to a gabled roof, have been advocated in
the past. This usually is achieved by planting central
rows with tall trees and flanking both sides with shorter
trees or shrubs. In most cases, this design is less efficient, requiring more land but not necessarily providing
increased wind protection. However, these wider windbreaks provide valuable wildlife habitat benefits and
are an appropriate design when wildlife habitat is an
important objective of the landowner.
Windbreak design
In designing a windbreak, density should be adjusted
to meet the landowner’s objectives. In general, windbreaks with higher densities (multiple rows) are used to
protect wildlife, farmsteads, or homesites, while windbreaks with lower densities (one or two rows) are used
to protect crop fields.
A windbreak density of 40 to 60 percent provides the
greatest downwind area of protection and provides excellent soil erosion control. To get uniform distribution
of snow across a field, densities of 25 to 35 percent are
most effective, but may not provide sufficient density to
control soil erosion. Windbreaks designed to catch and
store snow in a confined area usually have three to five
rows of conifers or shrubs and densities in the range
of 60 to 80 percent. Farmsteads and livestock areas
needing protection from winter winds require multiple
row windbreaks with high densities. Typically, these
windbreaks have two or three rows of conifers, one
or two rows of tall hardwoods, and one or more rows
of shrubs. In these cases, wind speed reductions are
greater but the extent of protected area is smaller.
Open Wind Speed 20 mph
Deciduous 25-35% Density
H distance
from windbreak
5H
10H
15H
20H
30H
miles per hour
10
13
16
17
20
50%
65%
80%
% of open
wind speed
85% 100%
Open Wind Speed 20 mph
Conifer 40-60% Density
H distance
from windbreak
5H
10H
15H
20H
30H
miles per hour
6
10
12
15
19
30%
50%
60%
75%
95%
% of open
wind speed
Open Wind Speed 20 mph
Multi Row 60-80% Density
H distance
from windbreak
5H
10H
15H
20H
30H
miles per hour
5
7
13
17
19
25%
35%
65%
85%
95%
% of open
wind speed
Open Wind Speed 20 mph
Solid Fence 100% Density
H distance
from windbreak
5H
10H
15H
20H
30H
miles per hour
5
14
18
19
20
25%
70%
90%
% of open
wind speed
95% 100%
Figure 2. Wind speed reductions to the lee of windbreaks with
different densities. A) density of 25-35%, B) density of 40-60%, C)
density of 60-80%, D) density of 100%.
Windbreaks are most effective when oriented at right
angles to prevailing winds. The purpose and design of
each windbreak is unique; thus, the orientation of individual windbreaks depends on the design objectives.
Farmsteads and feedlots usually need protection from
cold winds and blowing snow or dust. Orienting these
windbreaks perpendicular to the troublesome winter
wind direction provides the most useful protection. This
usually is accomplished by planting windbreaks on the
north and west sides of the farmstead or feedlot.
Successful field windbreaks should be designed to fit
within the farming operation. Consideration should be
given to reducing wind erosion, providing crop protec-
tion, increasing irrigation efficiency and improving
wildlife habitat.
Field crops usually need protection from hot, dry
summer winds; abrasive, wind-blown soil particles;
or both. The orientation of these windbreaks should
be perpendicular to prevailing summer winds, usually
south or west. Windbreaks designed to protect fallseeded small grains like winter wheat may need protection from both summer and winter winds. To control
soil erosion, windbreaks should be planted to block
prevailing winds during the times of greatest soil exposure — usually winter and early spring. To recharge
soil moisture with drifting snow, windbreaks should be
placed perpendicular to prevailing winter winds.
Although wind may blow predominantly from one
direction during one season, it rarely blows exclusively
from that direction. As a result, protection is not equal
for all areas on the leeward side of a windbreak. As the
wind changes direction and is no longer blowing directly against the windbreak, the protected area decreases
(Figure 3). The use of multiple-leg windbreaks provides
a more consistent and larger protected area than a
single windbreak. Again, individual placement depends
on the site, wind direction(s), and design objectives.
Microclimate modifications
The reduction in wind speed adjacent to a windbreak
reduces upward transport of heat and moisture from
the soil surface. As a result, temperature and humidity levels in the sheltered zones usually increase and
evaporation and plant water loss decrease. These
changes contribute to conservation of soil moisture, improvement of crop water use efficiency and an increase
in crop yields in the protected zone.
Actual temperature modifications for a given windbreak depend on windbreak height, density, orientation, and time of day. Daily air temperatures within
10H leeward of a windbreak are generally several
degrees higher than temperatures in the open. Beyond
10H, air temperatures near the ground tend to be
slightly cooler during the day. On most nights, temperatures near the ground in sheltered areas are slightly
warmer than in the open due to the reduction in wind
speed and in the upward transfer of heat from the surface. In contrast, on nights when wind speeds are very
low, the reduction in wind speeds in shelter may lead
to greater levels of radiation cooling and sheltered areas
may be several degrees cooler than open areas. In early
spring and late fall, these conditions may lead to frost
in sheltered areas.
Early in the growing season, soil temperatures in
sheltered areas usually are several degrees warmer
Windbreak
indbreak - 1 Leg
Windbreak
indbreak - 3 Legs
NE
NE
Protected
Area
Protected
Area
SW
Figure 3. In areas with winds from many directions, multiple-leg
windbreaks or windbreak systems provide greater protection to the
field or farmstead than single-leg windbreaks.
than in unsheltered areas. Taking advantage of these
warmer temperatures may allow earlier planting and
more rapid germination in areas with short growing
seasons. In the area immediately adjacent to an eastwest windbreak, soil temperatures tend to be higher
on the south side due to heat reflected off the windbreak. On the north side, soil temperatures, especially
in the early spring, are lower due to shading by the
windbreak. These cooler temperatures reduce the rate
of snow melt, and, in more northern areas, may cause
problems with field access in early spring.
Relative humidity in sheltered areas is two to four
percent higher than in open areas. Higher humidity
decreases the rate of plant water use, so production is
more efficient than in unsheltered areas. However, if
the windbreak is too dense and humidity levels get too
high, diseases may become a problem in some crops.
Moderation of windchill is most important in farmstead and livestock windbreak situations where humans and other animals readily notice the effects of
cold winter winds. Livestock use less feed and suffer
less weather related stress when protected from winter
winds. Similarly, good winter protection for outdoor
work areas makes winter chores less stressful and reduces the risk of injury due to extreme cold.
Summary
Windbreaks reduce wind speed on both the leeward
and windward sides. The resulting reductions in wind
speed lead to moderation of the microclimate in these
sheltered zones. With careful planning, and in consultation with local professionals, these changes in microclimate can be used to create desirable environments
for growing crops, raising livestock, managing snow and
protecting living and working areas. Windbreaks also
provide critical wildlife habitat in a landscape dominated by agricultural crops.
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the
U.S. Department of Agriculture, Elbert Dickey, Interim Director of Cooperative Extension, University of Nebraska,
Institute of Agriculture and Natural Resources.
It is the policy of Cooperating Agencies not to discriminate on the basis of sex, age,
handicap, race, color, religion, marital status, veteran's status, national or ethnic origin or sexual orientation.
This series of windbreak publications is jointly sponsored by the University of Nebraska, USDA Natural Resources
Conservation Service, USDA Forest Service, North Dakota State University, and the Forest Stewardship Program of
The Nebraska Forest Service. Its goal is to encourage the proper management of all our woodland resources.